Apply all the patches in the stack leading up to (but not including)
this one, and the AC_REQUIRE tree ends up putting the expansion of
_LT_ENABLE_INSTALL before the code that sets the shell variables it
examines.

We should think about adding a test to check whether `configure' has the
order correctly here (plus maybe documentation and AC_BEFORE instances
in case there are ordering issues hidden here); but I'd like to
understand the problem first.